Distribution transformers range widely in power delivery capability and physical size. Ratings are as small as 1.5 kVA and as large as 500 kVA or up to `small power` ratings of 2500 or 3000 kVA. The function of a distribution transformer is to reduce voltage on the medium utility system of from 2400 to 35000 down to utilization voltages of from 120 to 600 volts.
Transformers may be single phase devices or may be three phase devices, or may consist of single phase devices interconnected to supply three phase power. Distribution transformers can be oil-immersed, to provide better insulation and cooling or may be "dry-types" which are air-cooled, but will therefore be physically larger and require more weather protection than sealed, oil-immersed transformers.
Transformers generally contain two or more electrical circuits, primary and secondary windings, consisting of multiturn coils of electrical conductors that are interlinked by means of one or more magnetic circuits or cores. Cores typically consist of a plurality of ferromagnetic laminations that are stacked together to form a closed loop, surrounding and coupling magnetically the primary and secondary windings. Cores may be manufactured either from mutually overlapping or abutting individual laminations or from a continuous strip of magnetic sheet material wound around a mandrel to form a closed circuit. The magnetic and electric circuits are combined either by assembling the cores around pre-wound primary and secondary coils or by winding the conductor coils around one or more legs of the closed magnetic circuit. Examples of distribution transformers are disclosed by the following United States patents, although none of them relates to a solid state distribution transformer: U.S. Pat. No. 5,353,494, Oct. 11, 1994, "Method for Assembling a Distribution Transformer With Conforming Layers"; U.S. Pat. No. 5,202,664, Apr. 13, 1994, "Three Phase Transformer With Frame Shaped Winding Assemblies"; and U.S. Pat. No. 5,566,443, Oct. 22, 1996, "Methods of Making Power Distribution Transformers."
Conventional distribution transformers suffer from several undesirable characteristics:
1) they may require mineral oil or other liquid for cooling and as a dielectric medium or may require ventilation to the ambient air for cooling; PA1 2) the output voltage is a function of the input voltage and output current, and there is no provision to regulate the voltage or to compensate for power quality problems such as load harmonics, power factor or DC offset; and PA1 3) losses associated with energization of the core are present at all times and are independent of load. Efficiencies will approach zero for very small loads and will peak when supplying about 50% of nameplate rating. Lightly loaded transformers, therefore, are very inefficient.
In order to overcome some of these difficulties, previous researchers have proposed versions of a solid state transformer. See P. Reischi, Proof of the Solid State Transformer, EPRI TR-105069, Project 8001-13, Final Report, August 1995; and G. Venkataramanan, et al., AC-AC Power Converters for Distribution Control, presented at the NSF Symposium on Electric Power Systems Infrastructure, Washington State University, Pullman, Wash., Oct. 27-29, 1994. Most of this work appears to have been based on the topology depicted in FIG. 1, wherein bidirectional switches s1 and s2 are switched in a complementary fashion such that the voltage across s2, denoted v.sub.m, is equal to either the input voltage, v.sub.in, or 0. If the ratio of the time s1 is "on" (i.e., closed) to the total switching period is denoted k, then the effective value of v.sub.m is kv.sub.in, which is the primary mechanism by which the voltage transformation is achieved. The L and C elements serve to filter out the high frequency switching harmonics from v.sub.m.
The approach depicted in FIG. 1, however, is laden with difficulties. For example, the topology depends on complementary switching of switches s1 and s2. If, for example, s2 is closed ever so slightly before s1 is opened, the source voltage V.sub.in will be shorted, thus leading to a current spike. If s2 is closed slightly after s1 is opened, then for that instant there will be no path for the current in the filter inductor to flow, leading to a voltage spike. Since physical devices do possess an impedance and are not ideal switches, this approach is physically possible to implement but is nevertheless problematic and subject to high switching losses. Another disadvantage of this design is that, at the voltage levels needed to implement a distribution level transformer, semiconductors with appropriately high voltage ratings are not available. Series-connected devices must be used for this reason, but such devices are problematic in that device voltage ratings can be easily exceeded unless extreme care is taken to insure voltage sharing during switching transients. A further disadvantage is that, when implemented as a single stage, the semiconductors must be able to withstand both full primary voltage and full secondary current--a very expensive arrangement. Avoiding this difficulty requires using multiple cascaded stages but this can be problematic from a control perspective. In addition, the transformer depicted in FIG. 1 does not offer magnetic isolation. For these reasons, solid state transformers based on the design depicted in FIG. 1 do not appear to be practical.
The following United States patents disclose various aspects of a solid state transformer, but none relates to a solid state distribution transformer capable of handling voltage levels encountered by distribution transformers: U.S. Pat. No. 5,510,679, Apr. 23, 1996, "Reverse Phase-controlled Dimmer with Integral Power Adjustment Means"; U.S. Pat. No. 5,270,910, Dec. 14, 1993, "Neon Light Box"; U.S. Pat. No. 5,038,081, Aug. 6, 1991, "Reverse Phase-controlled Dimmer"; U.S. Pat. No. 4,204,237, May 20, 1980, "Solid State Transformer Differential Relay"; U.S. Pat. No. 4,071,378, Jan. 31, 1978, "Process of Making a Deep Diode Solid State Transformer"; and U.S. Pat. No. 4,024,565, May 17, 1977, "Deep Diode Solid State Transformer."